A psychiatrist said any intent by an Adelaide teenager to murder another was likely to have been impaired by alcohol and cannabis.

Forensic psychiatrist Dr Craig Raeside has given evidence to the South Australian Supreme Court.

On trial is a teenager who was 17 at the time of the shooting death last New Year's Eve.

He is charged with the murder of Lewis McPherson, 18, at Warradale last year.

The prosecution is trying to prove intent to kill Mr McPherson or cause grievous bodily harm.

But Dr Raeside told the court the accused was "highly intoxicated" from alcohol and a small amount of cannabis and it was likely he was more impulsive, with an impaired ability to predict the outcome of firing shots.

The accused's lawyer Bill Boucaut said: "It's one thing to intend to fire a handgun ... and another "to intend to bring about death or grievous bodily harm."

Prosecutor Tim Preston urged the judge to find the accused guilty of murder based on his actions before and after the shooting.

The accused did not give evidence on his state of mind, but the court heard he did not have a psychiatric illness.